342 F. JACOB AND E. L. WOLLMAN 



A. Limited Number of Prophages per Cell 



It is possible by indirect means to estimate the number of prophages 

 present in a lysogenic cell. On the one hand, it is known that when sensitive 

 bacteria are mixedly infected with different ratios of two mutant phages, the 

 same ratios are found in the progeny. On the other hand, it is known that 

 when UV-induced lysogenic bacteria are infected with a mutant phage, the 

 proportion in which the infecting type is foimd in the progeny increases as a 

 function of the multiplicity of infection. Assuming that after UV induction a 

 prophage plays the same role in phage multiplication as an infecting particle, 

 the number of prophages must be equal to the average multiphcity of infec- 

 tion for which a ratio of 1 : 1 is to be found in the progeny. The number of 

 prophages per bacterium may thus be evaluated as averaging three, which is 

 in good agreement with the average number of nuclei per bacterium in a 

 growing population (Jacob and WoUman, 1953). Similar conclusions have 

 also already been reached by Bertani (1953b) from experiments in which 

 non-inducible lysogenic bacteria were infected with a mutant phage and the 

 phage production analyzed as a fmiction of time. 



Lysogenic bacteria, therefore, appear to contain one prophage per nucleus. 

 Another line of evidence comes from the incompatibihty observed between 

 related prophages (Bertani, 1953a). The fact that, in most cases, lysogenic 

 bacteria are able to carry only one prophage of the same type, whereas they 

 may easily carry various imrelated prophages, suggests that, for a given 

 type of prophage, there must exist in a bacterium only a very small number 

 of specific sites saturated by the prophages. 



B. Chromosomal Location of the Prophage 



The decision as to whether the prophage is a cytoplasmic or a chromosomal 

 structure can only be obtained through experiments of genetic recombina- 

 tion. Such experiments are made possible by the existence, in strain K12 of 

 E. coli, of sexual processes discovered by Tatum and Lederberg (1947). 

 Wild-type strains of E. coli K12 are lysogenic and carry a prophage called A. 

 Nonlysogenic clones can be isolated by treatment such as exposure to heavy 

 doses of UV light (Lederberg, 1951). The genetic behaviour of the lysogenic 

 character may therefore be analyzed in crosses between lysogenic (ly^) and 

 nonlysogenic {ly~) bacteria. 



The first experiments designed to analyze the behaviour of the ly character 

 were carried out by Lederberg and Lederberg (1953) and by Wollman (1953), 

 using a system in which only one recombinant is formed per million of each 

 parental type. Although such a low frequency of recombination makes diffi- 

 cult any quantitative work, valuable information was obtained. Li certain 

 crosses, the A lysogenic character segregates among recombinants and 



